The main resource in a WCDMA downlink is the carrier power of the base station. The maximum carrier power limits the number of users that can be served, the service quality as well as the coverage of the base station. Each connection needs sufficient dedicated channel power to meet its associated quality requirement in terms of block error rate and thus provide acceptable perceived quality of service to the end user. Nevertheless, it is also important to utilize the power efficiently and not use more power than necessary, and therefore the transmitter power in the base station is regularly updated.
In WCDMA, fast power control is standardized for both up- and downlink. 3GPP, Physical layer procedures (FDD), Technical Specification TS 25.214. The user equipment (UE) sends transmitter power control (TPC) commands, i.e. ‘power up’ or ‘power down’ indications, to the network. These commands are used in the base station to update the dedicated power of the UE. The default algorithm is to step-wise update the power, using the TPC command to define whether the new power value is to be the previous power value plus or minus a fixed power step size. Provided that saturation does not occur, the power control command is always granted. There are two options associated with the default power control algorithm, the first of which reduces the risk of misinterpreted TPC commands, and the second limits the power raise of the power control through a sliding window size and a threshold.
The standardized power control algorithms in 3GPP are primarily designed for situations when it is possible to fulfil all service requirements and the mutual interference can be compensated for. However, since the radio environment is time varying, situations may arise where there is not sufficient carrier power in the base station to fulfil the service requirements of all users and there is a risk for unstable system behavior. Gunnarsson, F. and Gustafsson, F., Power Control with. Time Delay Compensation, Proc. Vehicular Technology Conference, Boston, Mass., USA, September 2000. Wireless communication systems are generally provided with means for admission control and means for disconnecting services, but these are relatively slow and not designed for handling system instabilities. Therefore, there is a need for mechanisms that are able to handle this on a small time scale with fast actions.
Several alternative power control algorithms have been proposed. In U.S. Pat. No. 5,574,982 to Almgren, et. al., for example, a quality target is gradually reduced when the dedicated channel power is increased. Essentially, this means that users requiring high powers have to put up with lower quality.
International patent application WO 02/35731 A1 addresses the problem of diverging transmitter output power levels of two or more base stations with respect to a mobile station in macro-diversity communication. The respective base station transmitter output powers for the mobile station are adjusted in response to the power control instructions from the mobile station and the respective current base station transmitter output powers for the mobile station. The adjustments can be performed in fixed or continuous steps.
Step size adjustments based on TPC history, mobility speed and bit error rate (BER) probability is e.g. described in the following documents: International Patent Application WO 00/04649; European patent application EP 0 815 656 B1; and U.S. Pat. No. 6,311,070 B1.
Although the above solutions have resulted in better downlink power control mechanisms they are still associated with problems. A drawback of prior art power control is for example that there is a considerable risk of overallocating or temporary running out of transmitter power. Insufficient power resources also result in that all connections are “punished”, which makes the situation rather unpredictable for individual mobiles.
Accordingly, there is a need for an improved downlink power control method.